Field of the Invention
This invention relates to phonograph pickup heads. More particularly it relates to pickup heads for electric phonographs in which provision is made for varying the weight of the head.
It is well known that, during playback, a phonograph cartridge and its supporting tone arm rise and fall in response to a warped record. This results in generation of a signal, at the cartridge output which varies cyclically at a frequency, related to the cycle of undulation, and falling primarily in a range below 10 to 12 Hz. To minimize generation of this signal, and thus avoid its playback through the reproducing system, it is customary to provide the tone arm with a mechanical resonance between the natural compliance of the cartridge and the total mass of both cartridge and tone arm, both being referred to the stylus position, the frequency of which occurs in the range of 10 to 12 Hz. Then, at frequencies below the resonance frequency, the effect of the undulations is damped and the cartridge output is desirably reduced to zero, eliminating the undesirable "warp" signals.
Tests, using typical cartridge-tone arm systems for playback of warped records, show that warp-induced motion can result in inertial forces, due to cartridge-arm mass, of up to 0.5 grams; these forces react vertically at the stylus of the cartridge. In high quality equipment where vertical stylus forces of one gram are adequate, with flat records, to produce satisfactory tracking of the tone arm, it is necessary, when warp motion is present, to increase the vertical stylus force 50% to 1.5 grams to prevent mistracking of groove signal modulations.
It is well known in the art that, to minimize record and stylus wear, the vertical stylus force should be as small as possible. The stylus force required to prevent mistracking due to the inertial forces of warp motion can be reduced by reducing the cartridge-tone arm mass. It follows that it is desirable to have a minimum cartridge-tone arm mass, as referred to the stylus position, and that adjustment of cartridge mass is the most effective and direct method of gaining this end.
A survey of tone arms now in general use for carrying playback cartridges reveals that effective arm masses (referred to stylus position) range from 5 to 8 grams, for more recent designs, and from 8 to 16 grams, for older designs, excluding cartridge mass. Also, counter-balance in these designs is such that zero cartridge weight is required to compensate for proper vertical stylus force in the more recent designs, where 0 to 4 gram cartridge weights were required in the older designs.
The study also revealed that, for arms with effective mass values of from 5 to 8 grams, a cartridge having appropriate compliance and a weight of 2.5 grams would provide mechanical arm resonance at a frequency in the range of 10 to 12 Hz. To satisfy weight requirements for proper counter-balancing, it was found that the cartridge weight must vary from 2.5 to 4.0 grams.